1. Field of the Invention
The present invention relates to a method for forming an insulating film in a semiconductor device, particularly to a method for forming a silicon oxide (SiO.sub.2) film on a substrate by the thermal chemical vapor deposition method (hereinafter referred to as a thermal CVD method) using a mixed gas of ozone (O.sub.3) and tetraethoxysilane (TEOS).
2. Description of the Prior Art
In recent years, forming further piled multilayer and making patterns finer is desired according to further improvement of an integration level in a semiconductor device. Thus, improvements of coverage and planarization have been made against an insulating film covering interconnection layers. As a method to attain them, a method for forming an SiO.sub.2 film by the thermal CVD method using a gas mixture of O.sub.3 +TEOS is attracting attention. In this case, a gas mixture containing O.sub.3 at a fixed ratio in O.sub.2 is used as an O.sub.3 source.
However, the quality and shape of an SiO.sub.2 film formed by the above film forming method are greatly affected due to the type of a backing insulating film exposed at a depositing surface and the mixture ratio of a reaction gas.
When the backing insulating film is an SiO.sub.2 film formed by a plasma chemical vapor deposition method (hereinafter referred to as a plasma CVD method) using silane, the coverage and smoothness are considerably improved.
In particular, however, when a backing insulating film 3 shown in FIG. 1A is an SiO.sub.2 film formed by the pasma CVD method using a gas mixture of O.sub.2 +TEOS or a silicon nitride (SiN) film, an SiO.sub.2 film 4 formed on the backing insulating film 3, as shown in FIG. 1B, is hardly deposited on a corner 5 of an interconnection layer 2 or the like to cause a notch or hollow 6. Moreover the density of the SiO.sub.2 film 4 is deteriorated and it results in soak of exterior moisture into the SiO.sub.2 film 4.
Therefore, soak of exterior moisture or the like may cause the interconnection layer 2 formed on a silicon substrate 1 to corrode or a leak current of a transistor (which is not illustrated) to increase. Moreover, as shown in FIG. 1B, smoothness degrades because a ruggedness 7 occasionally breeds on a surface.
To solve these problems, there is known a method for forming a film under a condition in which O.sub.3 is contained in O.sub.2 at a low concentration, for example, at approx. 1%.
However, according to the result of an examination using FTIR (Fourier Transform Infrared Spectroscopy), it is found that a larger amount of moisture is contained in the formed SiO.sub.2 film and also organic matter or the like in a reaction gas remains in the film when an SiO.sub.2 film is formed at a low O.sub.3 concentration. Moreover, according to the result of an examination using TDS (Thermal Disorption Spectroscopy), it is also found that the desorbing amount of the residue is large. These factors may cause an interconnection layer to corrode or a leak current of a transistor to increase.
When a backing insulating film is an SiO.sub.2 film formed by the plasma CVD method using a gas mixture of O.sub.2 +TEOS or an SiN film, there is used a method such that a plasma of a oxygen gas is applied to the backing insulating film to reform the depositing surface of the backing insulating film and thereafter a film is formed on the backing insulating film. This method improves the smoothness of the SiO.sub.2 film formed by the thermal CVD method using a gas mixture of O.sub.3 +TEOS.
However, when a plasma is applied to the backing insulating film, an electrical charge build-up occurs and may cause a semiconductor device to be ruined through a dielectric breakdown or the like. Moreover, damage is induced on a silicon substrate due to ion bombardment and thereby a device such as a transistor may be deteriorated.